This invention relates to an apparatus comprising an optical densitometer for determining and evaluating color measurement strips on a printed test sheet lying on a coordinate measuring table.
In the state of the art of printing machines, the densities of ink or color balance in printing machines are established by numerous ink-dosing elements arranged across the width of the printing machine and the ink-dosing elements are individually adjustable by remote control adjusting devices. In order to generate the control signals for the automatic ink-dosing elements, color measurement strips are printed on production sheets and some of these production sheets are selected as test sheets. After printing, the test sheets are conveyed to a densitometer table having an optical densitometer mounted for traversing or scanning the color measurement strips. In state of the art systems, the signals from the optical densitometer are automatically processed to compare the measured ink densities to desired ink densities in order to remotely control the ink-dosing elements in the printing machine. See, for example, Schramm et al. U.S. Pat. No. 4,200,932 issued Apr. 29, 1980 for which a reexamination certificate issued Apr. 26, 1983. The major printing machine manufacturers sell systems similar to the system disclosed in Schramm et al. 4,200,932.
Although the ink feed controls for the printing machine are generally automatic, the selection of test sheets and the scanning of the color measurement strips on the test sheets is a repititious process. A laborious and sometimes error generating step in the process has been the proper alignment of the test sheets on the densitometer table.
The alignment problem is particularly serious in view of the various types of paper ranging from thin printing paper to thick cards. Card orders require the maximum utilization of the area available for printing and thus for card orders the positions and total area available for color measurement strips are highly restricted. In practice, the color measurement strips are always at different places for different printing orders or jobs depending upon the orientation of the printed text on the sheets. The scanning process is also adversely affected by curling of the test sheets so that the test sheets do not lie exactly flat on the coordinate measuring table, and by variation in illumination across the sheet.